gpio: altera: fix return value of altera_gpio_remove()

The remove callback never succeeds, which seems odd.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Tien Hock Loh <thloh@altera.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>